Final word on cell phones and cancer: Reply hazy, try again

A new long-term study from Denmark’s Institute of Cancer Epidemiology examined the medical histories of more than 350,000 mobile phone users over a period of eighteen years. The study’s conclusion? Mobile phone users are at no greater risk for developing cancer in the brain or central nervous system than people who don’t use mobile phones. The study has been published in the British Medical Journal.

The finding is no doubt reassuring to the billions of people who increasingly rely on their mobile devices in their everyday lives, and is one of many studies that have found no discernable link between mobile phones and cancer. However, it also seems to fly in the face of other high-profile warnings, including the World Health Organization, which recently listed mobile phones as a possible carcinogen. Concern about cell phone radiation remains very high in some locales — in fact, San Francisco has repeatedly looked at requiring mobile phone retailers disclose how much radiation their devices emit along with potential risks.

Should mobile users be concerned about radiation? And what does the new Danish study establish?

The potential risk

The argument that mobile phones could pose a cancer risk comes from the technology itself: Mobile phones emit non-ionizing radio frequency radiation when they’re in communication with nearby cellular equipment. It’s literally how the devices communicate with the cellular network. The amount of radiation emitted varies depending on the mobile technology involved, distance from cell towers, phone design, and a variety of other factors, but the levels have been consistently quite low throughout the entire history of mobile phones, and improvements in the technology have generally lowered emissions over time.

Unlike ionizing radiation — think X-rays and gamma rays — that are known to cause an increased risk of cancer by damaging DNA, the only known biological effect of exposure to radio frequency radiation is heating. One example of how radio frequency radiation can heat things is microwave ovens: They work by exciting water molecules in food. However, microwave ovens work at different frequencies and at far greater power levels than mobile phones. It’s not as if you could hold a mobile phone next to a glass of water to heat some tea. No one has ever been able to detect any change in body temperature as a result of cell phone use. Radio frequency radiation has never been known to damage DNA, cause cancer, or impact the cancer-causing effects of other known carcinogens.

The perceived risk comes not so much from the amount of radiation phones emit, but how people use their phones. Much of the time, they’re holding the devices right next to body tissue — including the brain — that can potentially absorb the radiation. And, as we all know, mobile technology has become extremely popular: Plenty of people spent hours a day with a cell phone seemingly glued to the side of their head. The concern is that, over time, there may be a cumulative effect from exposure to radiation from phones, particularly amongst users who spend a lot of time on the phone. The concern is particularly acute for children and and teens, whose brains are still undergoing significant growth and development.

Latest findings

The Danish study is actually an update on previous research that had already been published, and examines the health records of more than 358,000 Danes from 1990 to 2007. Overall, researchers found there were 10,729 cases of central nervous system tumors in the group, with almost no distinction in incidence between men and women. When researchers narrowed the pool to Danes who had used mobile phones for different periods for either 10+ years or 13+ years, they were left with 356 cases of gliomas (brain cancers) and 846 cancers of the central nervous system. They found no significant indication of relationship between an individual’s first subscription for a mobile phone or the likelihood or anatomical location of a tumor — that is, whether a tumor showed up in areas of the brain closest to where handsets are held.

The data for people who have been using cell phones for 13 or more years is the newest: The previous update to the study had followed 420,000 cell phone subscribers who had been using the devices for 10 or more years.

Although the Danish study includes a considerable number of people, that doesn’t mean its pool of mobile phone users is perfectly representative of Danish society as a whole, let alone the entire human race. All participants in the Danish study are over 30 years old, and another notable omission from the subject pool are corporate mobile subscribers who use their phones for business. Potentially, corporate users could be amongst the heaviest users of mobile devices.

Despite finding no correlation between cell phone use and cancer, the researchers do indicate continued follow-up studies are warranted to continue looking at the effects of long-term mobile phone use, as well as to determine what effect, if any, cell phone use may have in children.

How the findings stack up

The Danish study is just the latest that fails to find a correlation between cell phones and cancer, but it is not the largest. That’s actually the Interphone Study, conducted by a group of researchers from 13 countries. It also found no statistically significant increases in brain or central nervous system cancers associated with high levels of cell phone use. However, the Interphone Study is largely funded by the telecommunications industry, and has been criticized for excluding a number of groups from its results, including children and young adults, invalids, users of cordless phones, and even users with particular types of brain tumors. The Interphone Study did find a modest association between the risk of gliomas and extremely high levels of cell phone use, but the researchers felt those levels of cell phone use were unlikely to be accurate. And since the Interphone Study is a meta-study of other research projects, there’s no real way for the researchers to validate the data.

In 2006, a case-controlled study in Sweden claimed to have found a link between risk of brain cancer and heavy cell phone use, particularly in subjects who began using cell phones before age 20. However, a larger Swedish study did not find any increased risk of brain cancer amongst cell phone users between the ages of 20 and 69. And earlier this year, the CEFALO study looked at children in Denmark, Sweden, Norway, and Switzerland who had been diagnosed with brain tumors, and found children who had started using mobile phones at least five years ago were at no increased risk for brain cancer compared to children who had never regularly used mobile phones, and also found no increased risk of rumors in areas of the brain receiving the highest amount of exposure.

Why do study results vary so much?

Different studies can produce different results for any number of reasons, but leaving aside the possibility of errors in data collection and analysis, the most common reasons are probably participation bias and recall bias.

Participation bias is all about who is included in a study. To be statistically valid, researchers need a sufficiently large pool of participants randomly selected from a particular population—in this case, mobile phone users. That’s (much) easier said than done. Participation in medical research studies is almost always voluntary, which means that anybody who doesn’t want to participate for any reason — too busy, uncomfortable, or just doesn’t like the interviewer’s shoes — isn’t included. In studies of cancer and cell phone use, people who are diagnosed with brain tumors are more likely than healthy people to participate; similarly, folks who rarely use cell phones are less likely to participate than heavy cell phone users. If you want to debunk a study that’s drawing a conclusion about a large group or a population as a whole, one of the first things to consider is whether the study’s participants are truly representative of that group.

Recall bias is all about data collection. Researchers in mobile phone usage studies sometimes have access to usage data from mobile phone operators, but a surprising amount of data collection about usage habits comes by way of questionnaires and interviews, and there’s no way to verify whether information participants provide about their cell phone use is accurate. Magnifying the possibility of inaccurate data collection is the amount of time involved. Sure, you might remember how much time you spent on your cell phone this week, but how about how much time you spend on your cell phone in the middle of October 2004? People with brain tumors may also remember their cell phone use differently than people without brain tumors, and may tend to think they used their phones mostly on the same side of their head where their tumor was found, regardless of actual use.

The bottom line here is that while sophisticated statistical analyses and examination of the data might yield some complex numbers and reassuring significant digits after decimal points, nearly all the figures that appear in these studies are fuzzy in one way or another. To be sure, the researchers involved are highly skilled and are no doubt using sound methods to remove and account for various kinds of bias. But the studies don’t represent a straightforward tally, or offer a black-or-white yes-no answer. This fuzziness is essentially why the International Agency for Research on Cancer has listed mobile phones as a possible carcinogen (PDF)—alongside things like DDT, gasoline, coffee, and some types of pickled vegetables.

What about children?

Children’s brains and nervous systems are undergoing rapid development, which potentially puts them at greater risk than adults for developing brain cancer from cell phones, assuming there is any risk at all. For one thing, there’s just sheer physical size to consider: children’s heads are smaller than adults, which means their proportional exposure to a phone’s radiation is greater. Also, since today’s kids are being born into an already mobile world, their lifetime use of mobile technology is likely to far exceed that of today’s adults. In other words, if there are long-term risks of brain cancer from cell phone use, today’s children are most likely to show the results.

What can you do?

There is no evidence that using mobile phones—even very heavily—has any correlation to developing brain or nervous system cancer. So, in all probability, you can use your phone(s) any way you like and be just fine.

If you’d like to reduce potential heretofore undiscovered risks of brain and central nervous cancer from cell phone use, get a hands-free device. Keeping the cell phone away from your head massively reduces the amount of radio frequency radiation to the brain. (Although you might want to think twice about keeping an active phone in your pants pocket for hours on end.) Another option is to reduce call times: If you need to talk a bunch, use a landline. Cordless phones and Wi-Fi calling are also options, but those also emit radio frequency radiation, albeit almost always at levels far lower than cell phones.